[PATCH RFC v3 0/2] fhandle: expose u64 mount id to name_to_handle_at(2)

[Aleksa Sarai]

Now that we provide a unique 64-bit mount ID interface in statx(2), we
can now provide a race-free way for name_to_handle_at(2) to provide a
file handle and corresponding mount without needing to worry about
racing with /proc/mountinfo parsing or having to open a file just to do
statx(2).

While this is not necessary if you are using AT_EMPTY_PATH and don't
care about an extra statx(2) call, users that pass full paths into
name_to_handle_at(2) need to know which mount the file handle comes from
(to make sure they don't try to open_by_handle_at a file handle from a
different filesystem) and switching to AT_EMPTY_PATH would require
allocating a file for every name_to_handle_at(2) call, turning

  err = name_to_handle_at(-EBADF, "/foo/bar/baz", &handle, &mntid,
                          AT_HANDLE_MNT_ID_UNIQUE);

into

  int fd = openat(-EBADF, "/foo/bar/baz", O_PATH | O_CLOEXEC);
  err1 = name_to_handle_at(fd, "", &handle, &unused_mntid, AT_EMPTY_PATH);
  err2 = statx(fd, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &statxbuf);
  mntid = statxbuf.stx_mnt_id;
  close(fd);

Also, this series adds a patch to clarify how AT_* flag allocation
should work going forwards.

Signed-off-by: Aleksa Sarai <cyphar@xxxxxxxxxx>
---
Changes in v3:
- Added a patch describing how AT_* flags should be allocated in the
  future, based on Amir's suggestions.
- Included AT_* aliases for RENAME_* flags to further indicate that
  renameat2(2) is an *at(2) syscall and to indicate that those flags
  have been allocated already in the per-syscall range.
- Switched AT_HANDLE_MNT_ID_UNIQUE to use 0x01 (to reuse
  (AT_)RENAME_NOREPLACE).
- v2: <https://lore.kernel.org/r/20240523-exportfs-u64-mount-id-v2-1-f9f959f17eb1@xxxxxxxxxx>
Changes in v2:
- Fixed a few minor compiler warnings and a buggy copy_to_user() check.
- Rename AT_HANDLE_UNIQUE_MOUNT_ID -> AT_HANDLE_MNT_ID_UNIQUE to match statx.
- Switched to using an AT_* bit from 0xFF and defining that range as
  being "per-syscall" for future usage.
- Sync tools/ copy of <linux/fcntl.h> to include changes.
- v1: <https://lore.kernel.org/r/20240520-exportfs-u64-mount-id-v1-1-f55fd9215b8e@xxxxxxxxxx>

---
Aleksa Sarai (2):
      uapi: explain how per-syscall AT_* flags should be allocated
      fhandle: expose u64 mount id to name_to_handle_at(2)

 fs/fhandle.c                                       | 29 ++++++--
 include/linux/syscalls.h                           |  2 +-
 include/uapi/linux/fcntl.h                         | 81 ++++++++++++++-------
 tools/perf/trace/beauty/include/uapi/linux/fcntl.h | 84 +++++++++++++++-------
 4 files changed, 140 insertions(+), 56 deletions(-)
---
base-commit: c7b9563b58a77423d4c6e026ff831a69612b02fc
change-id: 20240515-exportfs-u64-mount-id-9ebb5c58b53c

Best regards,
-- 
Aleksa Sarai <cyphar@xxxxxxxxxx>